1. Field of the Invention
The present invention is generally concerned with testing any kind of optical system.
It is more particularly, but not necessarily exclusively, directed to testing any surface of an ophthalmic lens, whether it is a spherical or an aspherical surface, in order to evaluate its conformity with a nominal theoretical surface.
2. Description of the Prior Art
It is known that the performance of ophthalmic lenses depends in particular on their surfaces.
It has already been proposed to employ the RONCHI test to test an optical system using phase detection.
The known RONCHI test entails disposing at the point of convergence of the light wave from the optical system under test a diffraction of alternating array or grating opaque and transparent parallel lines and analyzing the system of fringes that can then be observed downstream of the array.
According to the laws of geometric optics, and ignoring diffraction phenomena, these fringes are always the same and depend only on the direction of the light rays constituting the wave concerned, without depending in any way at all on the nature of the medium that it has passed through. This being so, the fringes are characteristic of the aberrations of this wave.
Their slope represents the difference between the corresponding wave surface and the theoretical wave surface to which it should conform.
It then suffices to measure this slope at all points of the optical system under test, which is done in practise using the phase detection technique, and then to carry out an integration.
A method of this kind can be used to measure any defects of an optical system irrespective of its quality and has the advantage of being simple and of yielding very accurate results.
An optical device enabling this kind of method to be implemented is described in the article "Fringe scanning Ronchi test for aspherical surfaces" in the journal "APPLIED OPTICS", volume 23, number 20, of 15 Oct. 1984, and in the article "Phase measuring Ronchi test" in the same journal, volume 27, number 3, 1 Feb. 1988.
This optical device comprises, along an optical axis, emission means adapted to constitute a point source of coherent light, a holding station adapted to receive the optical system under test, a RONCHI grating, receiving means adapted to receive the image observable downstream of the RONCHI grating, and data processing means adapted to exploit this image by phase detection.
However, experience shows that as implemented this optical device cannot achieve the hoped for degree of accuracy.
Also, it cannot be used to test all optical systems or surfaces, and in particular it cannot be used for absolute measurement of the radius of a spherical object.
A general object of the present invention is an array-based optical device with significantly improved accuracy enabling any optical system or surface to be tested.